In mobile communication networks, the radio interface comprises layers 1 and 2 of the Open System Interconnection (OSI) reference model. Layer 1 is also called physical layer, while layer 2 is sometimes referred to as link layer. Mapping between transport channels and physical channels, spreading and modulation operations, power control and handover mechanisms are typical layer 1 functions. Layer 2 functions, on the other hand, provide the means to transfer data between network entities and to detect (and possibly correct) errors that may occur in layer 1.
In the specifications of modem Radio Access Networks (RANs), such as the Universal Mobile Telecommunication Services (UMTS) Terrestrial RAN (UTRAN) and its Long-Term Evolution (LTE) extension, layer 2 is split into several sub-layers. Each sub-layer is defined by a dedicated radio interface protocol, including the Medium Access Control (MAC) protocol, the RLC protocol and the PDCP. In brief, MAC processing includes mapping between logical channels and transport channels and error correction, RLC processing provides segmentation, concatenation and retransmission services, and PDCP processing comprises compression operations and access stratum security procedures.